1. Field of the Invention
The present invention relates to a communication method and system for mobile communication. The present invention further relates to a communication method for mobile communicatin that employs a cellular system (which means the service area is covered by many circular or hexagonal shape small zones uniformly) which enables recovery of the communication quality when deteriorated as a result of movement of the mobile unit which is engaged in communication, and also to a system for this communication method.
More specifically, the present invention provides a communication method and system which are superior in the frequency effective utilization factor, communication quality, radiotelephone control capacity, etc.
2. Description of the Related Art
In mobile communication effected in a wide service area, a system in which a single radiotelephone base station covers the entire area to communicate with a mobile unit which is within the service area is generally called "large zone system". On the other hand, in "cellular system", the service area is divided into a plurality of small circular or hexagonal shape areas in each of which a single radiotelephone base station is installed, and a mobile unit which is present in each area effects communication with the ratiotelephone base station installed therein.
The conventional cellular system is adopted, for example, in the mobile telephone system of NTT (Nippon Telegraph & Telephone Corp.) which is now in commercial service. In this case, when a mobile unit which is loaded on an automobile moves away from the radiotelephone base station with which it is in communication, for example, when the distance from the radiotelephone base station is greater than a distance of from 5 to 7 km, the electric field intensity of the received radio wave lowers and, therefore, the speech quality deteriorates. For this reason, in the cellular system, radiotelephone base stations are installed in the service area at regular intervals of from 10 to 12 km so that, in the above-described case, another radiotelephone base station is always present near (within from 5 to 6 km) the current position of the automobile, thereby enabling the communication to continue between the new radiotelephone base station and the mobile unit by use of another radio channel.
In the NTT system, a radiotelephone control station for controlling setting and release of the radiotelephone speech channel is installed to control a large number of radiotelephone base stations and mobile units, and the radiotelphone control station is connected to a public telephone network through a gateway exchanger that serves as an interface. When the speech quality has deteriorated, the radiotelephone control station instructs a plurality of radiotelephone base stations around the mobile unit concerned to receive the radio waves transmitted from the mobile unit and, when it is judged that a desired level of speech quality can be maintained if a new radio channel is set up between the mobile unit and a specific one of the instructured radiotelephone base stations, the radiotelephone control station effects control such that a new channel is set up between the mobile unit and the specified radiotelephone base station.
FIG. 1 is a schematic block diagram of a conventional system that performs the above-described operation. The prior art will be explained below with reference to FIG. 1.
Referring to FIG. 1, four circular zones (cells) 14A, 14B, 14C and 14D each having a radius of about 5 to 7 km are defined as service areas of a mobile telephone system. It is assumed that a mobile unit 15 loaded on an automobile is in communication with a radiotelephone base station 13A in the zone (cell) 14A. Since the automobile is traveling within the zone (cell) 14A toward the zone 14C, the relative distance between the radiotelephone base station 13A and the mobile unit 15 is increasing. Assuming that the radio communication is continuing and the automobile has moved into the zone (cell) 14C from the zone (cell) 14A, the distance between the radiotelephone base station 13A and the mobile unit 15 is longer than a distance of 5 to 7 km, so that the electric field intensities of radio waves respectively received by the radiotelephone base station 13A and the mobile unit 15 lower below a predetermined transmission quality level.
The condition of quality deterioration is constantly supervised by a radiotelephone control station 12. When the quality lowers below a predetermined level, the radiotelephone control station 12 requests radiotelephone base stations 13B, 13C and 13D adjacent to the radiotelephone base station 13A to measure the level of quality of the radio channel (assumed to be the channel CH1) which is being used for the communication between the radiotelephone base station 13A and the mobile unit 15. At the request of the radiotelephone control station 12, each of the radiotelephone base stations 13B, 13C and 13D tunes its own radio channel searching receiver (not shown) to the channel CH1 to receive the signal and reports the condition of the received signal to the radiotelephone control station 12.
Receiving the report, the radiotelephone control station 12 makes a comparison between the electric fields E.sub.B, E.sub.C and E.sub.D respectively received by the radiotelephone base stations 13B, 13C and 13D. When it is confirmed that E.sub.C &gt;E.sub.B, E.sub.C &gt;E.sub.D and E.sub.C satisfies a predetermined level of transmission quality, the radiotelephone control station 12 regards the automobile as having moved into the zone (cell) 14C from the zone (cell) 14A and cuts off the radio channel CH1 used in the zone (cell) 14A and initiates the procedure of allowing use of an unused channel (assumed to be the channel CH10) among radio channels available for the radiotelephone base station 13C in the zone (cell) 14C in place of the channel CH1, that is, the control station 12 beings the operation of channel hand-off (switching) during communication.
The following description will be made with reference to Yoshikawa et al. "Mobile Telephone Control", Telecommunication Research Institute of Nippon Telegraph & Telephone Public Corp., Research and Practical Application Report Vol. 26, No. 7, page 1885, 1977.
1) The channel switching signal is transmitted from the radiotelephone control station 12 to each radiotelephone base station 13 through a control line that is included in each transmission line 16, while a radio speech channel is used to transmit the channel switching signal from each radiotelephone base station 13 to the mobile unit 15.
2) The channel switching signal is sent to the mobile unit 15 from the radiotelephone base station 13 with which the mobile unit 15 has heretofore communicated, for example, the radiotelephone base station 13A, while a radio connection test tone is sent to the mobile unit 15 from a radiotelephone base station 13 to which the radiotelephone base station is to be switched, for example, the radiotelephone base station 13C.
3) If the radio connection test tone cannot be received by the mobile unit 15, the speech channel is switched back to the old speech channel set up between the mobile unit 15 and the radiotelephone base station 13A so as to enable the call to continue.
The speech channel switching that is presently used in NTT is effected in accordance with the above-described procedures 1) to 3). As will be clear from these descriptions, the user of a mobile telephone may experience the following problems:
(a) Since the control signal (a digital signal of 300 bits/sec. in this case) for switching in the procedure 1) appears in the output of the receiver in such a way that it is inserted into the speech channel after the signal from the other party has been cut off, the control signal mixes in the speech channel in the form of an audible sound of about 300 Hz, thus causing speech disconnection during the reception of the control signal.
(b) During the transmission test in 2), no noise mixes in the speech channel but no sound is available, so that the other party's voice is not transmitted to the user of the mobile telephone and the user's voice is not transmitted to the other party, either (speech disconnection).
The duration of speech disconnection due to the above-described reasons a) and b) is said to be from 0.7 to 0.8 seconds. On the other hand, the radiotelephone control station 12 instructs the radiotelephone base station 13C through the transmission line 16C therebetween to initiate communication withthe mobile unit 15 by use, for example, of the channel CH10. Since this instruction is executed at the same time as the above-described connection test, the radiotelephone base station 13A terminates the communication with the mobile unit 15 at this moment and the radiotelephone base station 13C initiates communication with the mobile unit 15 instead. In addition, the radiotelephone control station 12 requests a gateway exchanger 19 that serves as an interface between it and a public telephone network 10 to switch speech path switches SW provided in the gateway exchanger 19 for connecting each radiotelephone base station 13 to the public telephone network 10 so that the radiotelephone base station 13A is switched to the base station 13C. More specifically, in the speech path switch group SW shown in FIG. 1, the switch A-4 is turned off (indicated by the blank triangle), while the switch C-4 is turned on (indicated by the black triangle).
By the above-described operation, the communication between the mobile unit 15 used in the automobile and any telephone set in the public telephone network 10 is continued no matter to which cell among the zones 14A, 14B, 14C and 14D the automobile moves.
Thus, the user (caller) is given the technical guarantee that it is possible to make a telephone call at any time and to any place during traveling of the automobile within the service areas. The actual service makes full use of this technique.
It has become possible for the mobile communication that adopts such a cellular system to exhibit the following advantageous features, which cannot be attained by the large zone system:
(a) It has become possible to effectively utilize frequency by the cellular system wherein radio waves from a single radiotelephone base station are restrictively used within a narrow area and a large number of such radiotelephone base stations are disposed in a service area so as to use the same frequency repeatedly.
(b) The advent of digital synthesizers has enabled a large number (as many as several hundreds) of radio channels to be used for mobile units while being switched over from one to another. Further, since techniques of setting up and controlling radio speech paths between these large number of mobile units and radiotelephone base stations have been established, it becomes possible to make a large contribution to the effective utilization of frequency mentioned in the paragraph (a).
(c) Since radiotelephone control techniques required to efficiently set up and control radio speech paths for a large number of mobile units at the time of originating or receiving calls have been established, it has also become possible to make a large contribution to the effective utilization of frequency mentioned in the paragraph (a). In addition, it has become possible to effect channel hand-off during communication which is needed when a mobile unit which is engaged in communication moves from one cell to another.
However, the conventional system illustrated in FIG. 1 suffers from the problems that the users put up with inconveniences and no satisfactory service can be provided since insufficient or no technical measures have heretofore been taken against the inconveniences. Accordingly, the system has been needed to promote further effective utilization of frequency and serviceability.
These problems will be explained below:
i) It is necessary in order to achieve effective utilization of frequency to reduce the radius of each cell in the cellular system; however, if the cell radius becomes excessively small, the probability that a mobile unit which is engaged in communication will pass one cell to move into another cell increases. In consequence, the necessity for changing radio channels allotted to the respective cells occurs frequently when the mobile unit moves through cells from one to another and, at this time, both a radiotelephone base station concerned and the mobile unit must change the old radio channel for a new radio channel. Hitherto, the change of channels has been effected in the radio radiotelephone control station 12 (see FIG. 1); however, the change of channels causes instantaneous disconnection of communication and thus deteriorates the communication quality.
ii) There has heterofore been a prior art wherein mobile units which have different transmission power levels are introduced into the same system so as to operate as devices in the same one system. In this prior art, for example, mobile units (having a transmission output of 5 W in the case of the NTT mobile telephone system) which are loaded on automobiles and portable telephone sets (having a transmission output of 1 W in the case of NTT) which the users can carry along outdoors to access radiotelephone base stations for the mobile telephone system are used in the same system and this enables construction of an economical system since radio equipment that is accommodated in the radiotelephone base stations can be used in common.
From the viewpoint of effective utilization of frequency, however, since establishment of rules for reusing the same frequency is complicated, the advantage of the effective utilization of frequency is lowered. In addition, since two different levels of transmission power are used, the possibility of occurrence of interferences with other mobile units increases. Prevention of these problems results in an increase in the cost and also causes the effective utilization of frequency to be impaired.
iii) As the size of cells becomes smaller (so-called microcell), one cell which is covered by one radiotelephone base station overlaps more with another cell which is covered by an adjacent radiotelephone base station or another base station which is adjacent thereto. Therefore, when the prior art is employed as a radiotelephone control technique, there is a possibility that it will become impossible to control the system.
More specifically, the radio wave propagation characteristics are greatly affected (i.e., the propagation loss is increased) by topography and buildings within one cell. As the size of cells is further reduced for the purpose of effectively utilizing frequency and the range of each cell decreases (a radius of 1 km or less), the effect of topography and buildings increases in inverse proportion to it. It is also necessary to employ transmitters of relatively high level for radiotelephone base stations and mobile units used in the system in order to ensure satisfactory communication even in a place where the radio wave propagation characteristics are affected by topography and buildings. This means that, in a place where there is no effect of topography and buildings, a remote radiotelephone base station and a mobile unit which is within another cell can communicate with each other.
Accordingly, the primary concept that each cell is administered by a single radiotelephone base station and a wide plane which is defined as a service area is covered with a large number of cells disappears, but a large number of cells overlap each other to form a service area instead.
As a result, to smoothly use a cellular system which is in such conditions, it is necessary with the prior art to frequently effect setting, change and release of radio speech paths, so that the capacity of the radiotelephone control unit is exceeded to a substantial extent. Accordingly, in actual practice it is impossible to ensure speech paths smoothly. Conversely, considerations have been given to how to cope with such situations when systems are constructed.
iv) In mobile communication, the radio wave propagation characteristics are affected by movement of a mobile unit and this causes the communication quality to vary to a substantial extent, so that in a place where the propagation loss of radio waves is high the communication quality may be lowered below the value required for the system. To solve this problem, various measures, for example, diversity technique, have been taken, but all these measures have problems, that is, a rise in the cost of the devices and impairing of the effective utilization of frequency.
Communication disconnection caused by movement from one cell to another during communication is considered to be a problem of a kind of communication quality and it has heretofore been necessary to find a solution in order to ensure the required quality.
v) No measure has been taken against communication traffic variations in the system.
Communication traffic of public land line communication is very small in the middle of the night and at early hours in the morning and high traffic peaks are found around 10 a.m. and at from 2 to 3 p.m. In the case of mobile telephone system, a high traffic peak is found at from 5 to 6 p.m. If a communication system is designed so that it functions satisfactorily at the time of the maximum traffic, devices that constitute the system are unused when the traffic is not congested, which results in an increase in the cost. If a communication system is constructed so that it is conformed with a small-traffic state, the cost is considerably low, but when the traffic reaches a maximum, the system cannot be used satisfactorily, so that serviceability is inevitably lowered.
In addition, the prior art lacks the concept that constituent facilities in the system are effectively utilized in such a way that, when the communication traffic is not congested, unused equipment in the system is effectively utilized to provide high-quality service, whereas, as the traffic increases, the service shifts to the ordinary service. It is considered that one of the reasons for this resides in that the system cost rises rapidly if it is intended to solve this problem by the prior art.
In the case where all transmitter-receivers in a radiotelephone base station are in use, for example, the conventional system is incapable of processing even an extremely short and fixed-form signal such as a position registration signal that is sent from a mobile unit, which constitutes a technical restriction against the progress toward smaller cells.
vi) Position registration of a mobile unit which is going to perform communication has heretofore been executed by registering only data which is received by one radiotelephone base station among data which are received by a plurality of base stations at the same point of time. For this reason, there are cases where it is impossible for the mobile unit to receive an incoming call due to a failure in position registration in systems wherein position registration is successively revised at a considerably high frequency as in the case of mobile communication with a mobile unit moving at high speed or in systems wherein the method of position registration is restricted due to the effective utilization of frequency.
More specifically, in the case where the radiotelephone base station is provided with transmitter-receiver equipment for only one channel, the channel must be used in a time-division manner for both control and speech. In addition, when the transmitter-receiver is in communication with one mobile unit, if there is a request for position registration from another mobile unit which is in the same cell, marked adverse effects are experienced.
vii) The technique for providing mobile communication service that employs wide-band signals has not yet satisfactorily been completed; therefore, the users have heretofore suffered much inconvenience.
Hitherto, many mobile communication services have been used for telephone services, but high-speed data signals or the like which use wide frequency bands have not substantially been used. This is because in mobile communication the radio wave propagation characteristics change greatly as the mobile unit moves and there have been no technique of receiving wide-band signals effectively.
viii) In conventional land mobile communication, estimation of a direction of movement of a mobile unit which is engaged in communication has not been practiced due to technical difficulties except for some special cases. For this reason, it is impossible to obtain effective information such as radio telephone traffic conditions in an area to which the mobile unit is moving. Therefore, there are problems to be solved foe effective utilization of frequency or traffic administration.
ix) Instantaneous transmission disconnection occurs at the time of switching over speech channels from one to another which is needed when the mobile unit moves from one cell to another or when it is moving within one cell. This also constitutes a major obstacle to achievement of an effective cellular system.
The speech channel switching method practiced by NTT, which has been explained with reference to FIG. 1 involves the disadvantages that the speech is momentarily (for from 0.7 to 0.8 seconds) disconnected when radio channels are switched from one to another and further a part of control signal (300 bits/sec.) mixes in the speech signal to generate noise that grates on the user's ear. When the call is made by voice, even if instantaneous speech disconnection or mixing of noise occurs, a missing part of the speech can be recovered by asking the other party to say it again; in such a case, therefore, no serious problem is experienced. However, in the case where a facsimile terminal which is loaded on an automobile is used to send or receive information, if channel hand-off is effected during an operation of the facsimile terminal, there has heretofore been experienced the problem that, in the case, for example, of a 1-minute facsimile system, a black (or white) line appears on that portion of the surface of a sheet of paper which is located at 0.8/60 of the sheet, thus greatly deteriorating the quality of received picture. In the case of data communication, signals that correspond to about 1,000 bits are missed when data signals are transmitted, for example, at 1,200 Bd, and it is therefore necessary to resend the data.
It should be noted that there are some methods of removing grating noise, e.g., one method wherein no sound is generated when channels are switched over from one to another and another method wherein a signal which is out of the band for the speech signal is used as a control signal. These methods enable removal of grating noise but cannot eliminate the transmission disconnection. Therefore, the prior art methods are utterly ineffective to remove adverse effects on facsimile systems and data signals.